1. Field of the Invention
The present invention relates to video and audio systems. More specifically, the present invention relates to methods and apparatus for a combined video and audio system that provides three-dimensional, high resolution video images and corresponding audible sound over the full frequency range to a patient positioned in an MRI electromagnetic environment.
2. Description of the Related Art
A magnetic resonance imaging (MRI) scanner is a medical diagnostic apparatus employed to generate images of soft tissue of the human body which are not otherwise visible, for example, by the use of x-rays. In general, the MRI scanner includes a tunnel area which accommodates a patient lying, for example, in the supine position. The tunnel area is surrounded by a plurality of magnetic pickup coils. Radio frequency (RF) pulses generated by the MRI scanner enable the pickup coils to sense the changes in the energy state of the hydrogen atom protons of the human body. The changes in the energy state of the hydrogen atom protons are then captured, for example, on video images which are subsequently used for diagnostic purposes.
Generally, with respect to the use of MRI scanners, video and audio systems are employed for both (a) clinical and (b) research applications. In clinical applications, the concern is directed to anxious or claustrophobic patients who resist entering the tunnel of the MRI scanner. The capability to adequately display visual information for viewing and to efficiently transmit audio information responsive to the full frequency range of the human ear for listening pleasure are important factors for relief for the anxious or claustrophobic patient. The existing method to achieve patient comfort through visual entertainment is dependent upon the use of liquid crystal display (LCD) projection screens for display of visual information. The existing method to achieve patient comfort through audio entertainment is dependent upon the use of inefficient apparatus that provides poor sound quality and frequency response, is heavy and requires a voltage of approximately (30-50) volts RMS of a received audio signal to generate sufficient sound amplitude to enable the patient to hear the music entertainment over MRI gradient noise.
In one visual entertainment method, the LCD screen contents are viewed by the patient with the assistance of reflecting mirrors. The reflecting mirror method suffers from certain limitations. Initially, the LCD projection screen is located outside the bore of the scanner tunnel. Thus, the position of the patient within the scanner tunnel can interfere with the line of sight to the LCD screen. Further, the level of ambient light within the MRI magnet room and the need for repeated adjustments of the overhead reflecting mirrors result in limited use of this method. Since the overhead reflecting mirrors extend the field of view and the LCD projection screen is outside the tunnel, the patient is aware of her surroundings. Thus, the possibility of being distracted by the external surroundings in addition to the interior of the tunnel further limits the usefulness of this technique for the reduction of anxiety and claustrophobia in patients.
In another visual entertainment method, an MRI video system provides video images in a magnetic field through a fiber optic medium and includes a non-magnetic video source for generating a video image. A composite reduction lens is employed for capturing and compacting the generated video image and a fiber optic bundle receives and transmits the compacted video image. A magnification lens intercepts the compacted video image from the fiber optic bundle and then magnifies and focuses the intercepted video image in a magnetic field. In this application, the LCD image is transmitted to the inside of the scanner tunnel to a set of goggles so that the total field of view of the patient is limited by the goggles.
Current MRI fiber optic systems that position the LCD screen within the scanner room (but outside the bore of the MRI scanner) are extremely useful and provide a definite advance in the art. Notwithstanding, certain features of this design could be improved. In particular, the length of the fiber optic bundle employed to carry the video images from the LCD screen to the eyepiece for viewing by the patient is of concern. As with all transmission systems, a portion of the transmitted parameter is lost during transmission and the longer the transmission path, the greater the loss. For long fiber optic bundles, it is known that the loss of as much as forty percent (40%) of the transmitted video image can occur. This loss affects the resolution and brightness of the transmitted video image. Therefore, the resolution and brightness of the transmitted video image is limited by the length of the fiber optic bundle. Additionally, the longer the fiber optic bundle, the more cumbersome it is to carry the bundle and associated fiber optic equipment into and out of the MRI scanner tunnel.
A fiber optic bundle is comprised of a plurality of optical fibers. When an optical fiber is interrupted, the pixels of light of the transmitted image carried by the interrupted fiber are blocked. This situation results in dead pixels, e.g., black spots that appear on the video display. As the length of the fiber optic bundle is increased, the probability that individual fibers will be broken increases. Further, as the fiber optic bundle is bent and manipulated over a period of time, the number of broken fibers increases. An increasing number of broken fibers results in a greater number of black spots appearing on the video display. Eventually, the transmitted image becomes inadequate and distorted. Thus, long fiber optic bundles are not cost effective.
During an MRI examination, the patient is positioned upon an examination table which can be moved into and out of the MRI scanner tunnel. When lying upon the examination table within the scanner tunnel, the patient's head is positioned within a head coil. The head coil is arranged to surround the patient's head and to provide MRI images thereof. Advanced designs of MRI scanner head coils minimizes the distance between the patient's eyes and the top of the head coil. The limited distance between the patient's head and the head coil would be inadequate to accommodate the goggles employed by known MRI fiber optic systems that (a) position the image from the LCD display within the scanner tunnel or (b) employ a reflecting mirror over the patient's eyes.
The second use of video systems in MRI scanners is directed to research applications. In particular, video systems have been utilized to generate visual activation in MRI scanner studies of the visual cortex in the human brain. The visual activation, also known as functional imaging, typically includes audio or video brain stimulation utilizing computer generated images during an MRI scan of the brain. When, for example, a video image is displayed before the patient during the MRI scans, different sections of the brain will induce energy. The change in the energy level of the brain can be identified on the MRI images. These research activities are typically conducted by universities and research facilities. However, functional imaging is also useful to map specific areas around a tumor in the brain to determine, for example, the boundaries for surgical removal thereof.
The current state of video image display within the MRI magnet room in research applications consists of projection of the image onto a screen mounted on a wall or placed at the end of the imaging table. The projection is achieved within the magnetic environment by employing an MRI-compatible LCD screen. The video information is viewed by the patient with the aid of adjustable light reflecting mirrors. The utility of this method of visual activation is limited by the position of the patient within the scanner tunnel. Further, the level of ambient light in the MRI magnet room will effect the quality of the image that the patient sees on the screen. A high level of ambient light will cause the screen image to be washed out. Also, the time required to adjust the light reflecting mirrors with respect to the LCD screen is determined by the position of the patient inside the scanner tunnel. For functional magnetic resonance imaging, it is ideal to cover the entire patient field-of-view with the MRI screen or display.
The effectiveness of this method of visual activation is further reduced by an open field of view (e.g., the LCD screen is outside of the tunnel) which enables the patient to be aware of her surroundings. During functional imaging, the best results are achieved when the visual stimulus is controlled which is inconsistent with an open field of view. Further, this method of visual activation does not include the ability to generate three-dimensional (3D) images for patient viewing since the image is projected onto a single screen. The inability to create a condition is which the eye and brain perceive a 3D effect prevents virtual reality from being achieved.
It is known that the only reproducible and standard method for visual activation studies has been with the use of goggles fitted with single or multiple light emitting diodes (LEDs). Although this method has been effective in performing feasibility type functional MRI studies of visual activation, it is limited in that it represents a simple two-state photic stimulation paradigm (e.g., a two-state visual stimula). Advanced studies of the visual cortex will require the capability for implementation of visual activation paradigms more sophisticated than simple flashing lights. Further, the limited distance between the patient's head and modern head coil designs would be inadequate to accommodate the goggles employed in visual activation studies.
Typically there are two methods in which audio information can be transmitted to the patient. These methods include the use of (a) a magnetic transducer and (b) a non-magnetic transducer. A magnetic transducer or speaker incorporates a ferrous core in the construction thereof for transducing an electrical signal into sound energy by operating a diaphragm. Because of the ferrous core, the magnetic speaker must be located (20'-50') away from the patient positioned within the tunnel of the MRI scanner. This requirement is necessary so that the static magnetic field of the MRI magnet will not saturate the speaker magnetic core within the transducer and generate heat resulting in damage thereto. The entire magnetic speaker could be attracted and placed in motion by the MRI magnetic field and cause injury to the patient. Positioned at the output of the magnetic speaker is a non-magnetic transmitting medium such as a plastic tube which is employed to deliver the audio signals to a set of headphones used by the MRI patient as is known in the art.
The medium (e.g., plastic tube) utilized to transmit the audio signals from the magnetic speaker to the headphones must also be non-magnetic to avoid generating stray signals that interfere with the MRI magnetic field. Therefore, the plastic tube must be (20'-50') in length extending from the magnetic speaker to the headphones. One of the characteristics of the plastic material used to form the tube is that it tends to absorb the high frequency components of the audio signal. Thus, the frequency response is poor and the sound quality is muffled, unclear and annoying to the MRI patient.
The second method of transmitting audio signals to an MRI patient utilizes a non-magnetic speaker. An example of a non-magnetic speaker includes (a) an electret element or (b) a piezo ceramic (piezoelectric) transducer as is known in the art. These two non-magnetic speakers each serve as a transducer to convert electrical signals to sound energy, do not include a magnetic core and thus do not interfere with the MRI generated magnetic field. The electret element has a plurality of disk-shaped plates and three electrical terminals. The terminals include a positive, negative and biasing terminal. The positive terminal is connected to a first copper plate and the negative terminal is connected to a second copper plate in juxtaposition to the first copper plate. The biasing terminal is connected to a third copper plate positioned between the first and second copper plates. In operation, the positive and negative plates are energized at several hundred volts. A biasing voltage is applied to the third copper plate via the biasing terminal which causes the transducer to oscillate at a frequency within the range of 15 Hz to 30 KHz. The biasing voltage provides the amplification of the audio signal applied to the electret element as is known in the art.
The piezo speaker includes an element shaped in the form of a disk and comprised of, for example, ceramic material. When an audio signal is impressed across a piezo element, the transducer generates sound energy as is known in the art. Further, when a diaphragm is mounted over and communicates with the piezo element, the sound energy is amplified. Each of these transducer structures provides an audio speaker that can be employed to provide audio signals to the MRI patient within the scanner tunnel.
Thus, there is a need in the art for an improvement in combined video and audio systems for use with MRI scanners which provide high resolution video images with a three-dimensional effect, shortens the transmission paths that the video image and audio signals must travel, eliminates the problems associated with fiber optic bundles, is sized to fit within the limited space of modern head coil designs, is light weight and economical to produce, provides audio signals over the full frequency range and can be mounted and operated within the MRI magnetic field.